Charged particle beam (CPB) accelerators such as electron accelerators are known in the art. An electron accelerator applies a local electric field to a cluster of traveling electrons, accelerating the electrons through the structure. In this way, the electrons continuously or successively acquire energy until their total energy is many times their rest energy, and their velocity is very close to the velocity of light.
At Lawrence Livermore National Laboratory (LLNL), an electron accelerator known as the Experimental Test Accelerator (ETA) has been fabricated and tested. The ETA employs linear magnetic induction to accelerate electrons. The initial voltage pulse is formed by a coaxial Blumlein transmission line that is triggered by a sparked discharge from an energy storage and charging network. In the first of four sections of the ETA, the electron beam pulse is produced by an electron injector that consists of an anode-cathode and a series of magnetic accelerating units.
This beam pulse, or electron cluster, is fed into the second section, which is a post-accelerator that increases the electron energy up to the final desired value through a series of additional magnetic induction units. In the third section, the beam is then guided by a beam-transport unit into a fourth section, which in the case of the ETA was the experimental tank or test region. A more detailed discussion of the ETA may be found in the article "Accelerating Intense Electron Beams" published in Energy and Technology Review, Lawrence Livermore National Laboratory, September 1979, pages 16-24; this article is incorporated by reference into this specification.
The follow-on to the ETA at LLNL is the Advanced Test Accelerator (ATA), which is a linear induction electron accelerator. The already fabricated 200 meter ATA facility has an 85 meter linear accelerator, and consists of four major units: a power conditioning system, a 2.5 MeV electron injector, a 190 module 47.5 MeV accelerator followed by a beam transport pipe, and an experimental tank. The power conditioning system consists of all power supplies, capacitor banks, and pulse conditioning networks which ultimately provide the short, high-voltage pulses that drive the electron injector and accelerator modules. The injector is essentially a 2.5 MeV triode with a hollow anode through which a 10 kA electron beam is injected into the downstream accelerator sections.
The beam is guided magnetically through the accelerator consisting of 190 accelerating cavities (250 kV each). The electron beam, at full energy and still magnetically guided, enters an experimental tank that contains gas of various types and pressures. The accelerator parameters are as follows: 50 MeV, 10 kA, 70 ns pulse width (FWHM), and a 1 kHz repetition rate (rep-rate) during a 10-pulse burst. In addition, beam quality and pulse-to-pulse repeatability must be excellent. The unique features of the ATA are the 10 kA beam and the 1 kHz burst frequency. A more detailed discussion of the ATA may be found in the paper entitled "The Advanced Test Accelerator: A High-Current Induction Linac", LLNL paper UCRL-88312, by E. G. Cook, D. L. Birx and L. L. Reginato, dated Nov. 1, 1982; this paper is incorporated by reference into this specification.
The basic building block of the ATA accelerator is what is variously referred to as the induction unit, or the accelerator cell, or the accelerator cavity. The drive pulse via the two oil-filled cables connects to the metal structure surrounding the 20-inch outside diameter ferrite toroid. The cast epoxy insulator is the oil-vacuum interface, and the electron beam center line is through the center of the cell. Electrically, the cell may be viewed as a 1:1 transformer having a single, very tightly-coupled turn around the ferrite toroids as the primary, and the electron beam as the secondary turn. The accelerating voltage is measured across the one inch gap, while the electron beam sees and gains energy from the axial E-field (electric field) resulting from the flux swing in the ferrite toroids. ATA uses 190 of these induction cells or cavities, bolted together to form its 47.5 MeV accelerator.
Problems and shortcomings, however, exist in the present technology of accelerating charged particle beams. More specifically, charged particle beam (CPB) accelerators have produced high current and high particle energy charged particle beams such as electron beams, but the accelerators are often plagued with difficulties in guiding the beams, and more important, in damping out unwanted beam motion. For example, in a linear induction accelerator (often referred to as a "linac") where numerous accelerating cavities are used, a cavity mode-beam interaction, commonly referred to as the Beam-Break-Up (BBU) instability impresses transverse oscillations and displacement instablilities on the beam. Also, beams for finite rise and fall times present a time varying load to the accelerating induction cores of the cavities; this time varying load causes beam energy to vary slightly during the beam pulse. When steering magnet coils are used to guide the beam, this energy variation translates into a spatial sweep of the beam head and tail. Electron beam generators that use field emission cathodes are also susceptible to beam centroid movement due to time varying irregularities of the cathode emission surface. For many applications, transverse motion of the beam is an undesirable phenomenon that adversely affects beam propagation.
For a more thorough discussion of the beam dynamics and beam breakup instability, reference can be made to the following three documents, which are incorporated by reference into this specification: (1) "Further Theoretical Studies of the Beam Breakup Instability", Particle Accelerators, 1979, Vol. 9, pages 213-222, by V. K. Neil, L. S. Hall and R. K. Cooper; (2) "Transverse Resistive Wall Instability of a Relativistic Electron Beam", Particle Accelerators, 1980, Vol. 11, pages 71-79, by G. J. Caporaso, W. A. Barletta, and B. K. Neil; and (3) "Beam Dynamics in the ETA and ATA 10 kA Linear Induction Accelerators: Observations and Issues", LLNL document UCRL-85650, by R. J. Briggs, et al.
Attempts have been made to damp out the transverse motion of the beams, but these attempts have various disadvantages. U.S. Pat. No. 3,912,930, entitled "Electron Beam Focusing System", to Creedon et al. issued Oct. 14, 1975, discloses a wire which is positioned on a beam axis to establish a conducting path and anode from a cathode. From an external power source, voltage and current are applied to the wire, thus creating a circular magnetic field around the wire. The magnetic field concentrates and focuses the electron beam. This technique has the disadvantage of requiring that an external power source be attached to the focusing wire. Also, the asmuthally symmetric magnetic field created by the wire cannot damp the transverse motion of very high energy charged particle beams, such as found in the Advanced Test Accelerator at LLNL.
U.S. Pat. No. 3,209,147, entitled "Electron Lens Spherical Aberration Correcting Device Comprising a Current Carrying Wire Section on the Lens Axis", to Dupouy et al., issued Sept. 28, 1965, discloses an electron lens created by inducing a magnetic field in the vicinity of the wire by flowing a direct current through the wire and external power source. Again, this approach has the disadvantage of requiring the wire to be attached to an external power source, and, in essence relies on magnetic fields produced by the current carring wire.
U.S. Pat. No. 2,574,655, entitled "Apparatus for Focusing High-Energy Particles", to Panofsky et al., issued Nov. 13, 1951, discloses a magnetic lens, but this magnetic lens again does not damp out the transverse motions of a charged particle beam, such as used in the above referenced ATA.
U.S. Pat. No. 4,002,912, entitled "Electrostatic Lens to Focus an Ion Beam to Uniform Density", to Johnson, issued Jan. 11, 1977, discloses a plurality of wires which are at ground potential, and which produce an electrostatic field to redirect the ion particle beam; the ions are positively charged particles. A high voltage anode surrounds the wires. Focusing of the particle beam is accomplished by the potential difference existing between the anode and the wires. However, this design is undesirably complex and directed to deflecting ions rather than focusing them. Furthermore, it is not directed to the damping of transverse motion of a charged particle beam.
Therefore, problems of transverse oscillations of charged particle beams continue to persist, particularly in high energy particle accelerators such as the ATA at LLNL. Additionally, the prior art requires an external power source which is used to energize the focusing, stabilizing and guiding means. Thus, a need exists for an improved apparatus and method for attenuating these transverse oscillations.